Gallup’s approach to studying the design of the human penis is a perfect example of of “reverse-engineering” as it’s used in the field of evolutionary psychology. This is a logico-deductive investigative technique for uncovering the adaptive purpose or function of existing (or “extant”) physical traits, psychological processes, or cognitive biases.

(...) In addition, only our species has such a distinctive mushroom-capped glans, which is connected to the shaft by a thin tissue of frenulum (the delicate tab of skin just beneath the urethra). Chimpanzees, gorillas and orangutans have a much less extravagant phallic design, more or less all shaft. It turns out that one of the most significant features of the human penis isn’t so much the glans per se, but rather the coronal ridge it forms underneath. The diameter of the glans where it meets the shaft is wider than the shaft itself. This results in the coronal ridge that runs around the circumference of the shaft—something Gallup, by using the logic of reverse-engineering, believed might be an important evolutionary clue to the origins of the strange sight of the human penis.

(...) Thus, in a theoretical paper published in the journal Evolutionary Psychology in 2004, Gallup and coauthor, Rebecca Burch, conjecture that, “A longer penis would not only have been an advantage for leaving semen in a less accessible part of the vagina, but by filling and expanding the vagina it also would aid and abet the displacement of semen left by other males as a means of maximizing the likelihood of paternity.”

(...) So how did natural selection equip men to solve the adaptive problem of other men impregnating their sexual partners? The answer, according to Gallup, is their penises were sculpted in such a way that the organ would effectively displace the semen of competitors from their partner’s vagina, a well-synchronized effect facilitated by the “upsuck” of thrusting during intercourse. Specifically, the coronal ridge offers a special removal service by expunging foreign sperm. According to this analysis, the effect of thrusting would be to draw other men’s sperm away from the cervix and back around the glans, thus “scooping out” the semen deposited by a sexual rival.

(...) As predicted, the two phalluses with the coronal ridges displaced significantly more semen from the vagina (each removed 91 percent) than the “headless” control (35.3 percent). Additionally, the further that the phalluses were inserted—that is to say, the deeper the thrust—the more semen was displaced. When the phallus with the more impressive coronal ridge was inserted three fourths of the way into the vagina, it removed only a third of the semen, whereas it removed nearly all of the semen when inserted completely.

(...) In the second part of their study published in Evolution & Human Behavior, Gallup administered a series of survey questions to college-age students about their sexual history. These questions were meant to determine whether penile behavior (my term, not theirs) could be predicted based on the men’s suspicion of infidelity in their partners. In the first of these anonymous questionnaires, both men and women reported that, in the wake of allegations of female cheating, men thrust deeper and faster. Results from a second questionnaire revealed that, upon first being sexually reunited after time apart, couples engaged in more vigorous sex—namely, compared to baseline sexual activity where couples see other more regularly, vaginal intercourse following periods of separation involve deeper and quicker thrusting. Hopefully you’re thinking as an evolutionary psychologist at this point and can infer what these survey data mean: by using their penises proficiently as a semen displacement device, men are subconsciously (in some cases consciously) combating the possibility that their partners have had sex with another man in their absence.

(...) For example, one obvious criticism of the semen displacement theory is that men would essentially disadvantage their own reproductive success by removing their own sperm cells from their sexual partner. However, in your own sex life, you’ve probably noticed the “refractory period” immediately following ejaculation, during which males almost instantly lose their tumescence (the erection deflates to half its full size within 1 min of ejaculating), their penises become rather hypersensitive and further thrusting even turns somewhat unpleasant. In fact, for anywhere between 30 minutes to 24 hours, men are rendered temporarily impotent following ejaculation. According to Gallup and Burch, these post-ejaculatory features, in addition to the common “sedation” effect of orgasm, may be adaptations to the problem of “self-semen displacement.”